1. Field of the Invention
The present invention relates generally to apparatus and methods for mixing fluids, and more particularly, but not by way of limitation, to tube mixing of high density proppant laden gelled slurries for use in oil well fracturing.
2. Description of the Prior Art
One common technique for the stimulation of oil or gas wells is the fracturing of the well by pumping of fluids under high pressure into the well so as to fracture the formation. The production of hydrocarbons from the well is facilitated by these fractures which provide flow channels for the hydrocarbons to reach the well bore.
The fluids utilized for these fracturing treatments often contain solid materials generally referred to as proppants. The most commonly used proppant is sand, although a number of other materials can be used. The proppant is mixed with the fracturing fluid to form a slurry which is pumped into the well under pressure. When the fractures are formed in the formation, the slurry moves into the fractures. Subsequently, upon releasing the fracturing pressure, the proppant material remains in the fracture to prop the fracture open.
A typical slurry mixing apparatus such as that presently in use by Halliburton Company, the assignee of the present invention, includes a rectangular shaped tub having dimensions on the order of six feet long by four feet wide by three feet deep. In the bottom of the tub, lying parallel to the length of the tub, are two augers which keep the slurry in motion near the bottom of the tub and minimize the buildup of sand in the bottom of the tub. Sometimes, rotating agitators having blades with a diameter on the order of twelve to fifteen inches are provided near the surface of the slurry. Fluid inlet to these blender tubs may be either near the bottom, through the side, or into the top of the tub. Sand is added by dumping it into the top of the tub.
Slurry mixing is of primary importance during a fracturing job. The sand must be mixed with the fracturing fluid which often is a high viscosity gelled fluid. The resulting slurry is a high viscosity, non-Newtonian fluid which is very sensitive to shearing and can be difficult to thoroughly mix. The viscosity of the fluid depends upon the motion of the fluid and thus the viscosity of the slurry is to a significant extent dependent upon the manner in which the slurry is mixed. Most oil field service companies have few problems with present technology when mixing low sand concentration slurries, i.e., ten pounds per gallon or less sand concentration. Problems, however, start to arise when the sand concentrations exceed ten pounds per gallon. Sometimes very high sand concentrations are desired up to approximately twenty pounds per gallon. The problems encountered when mixing these very high density slurries include air locking of centrifugal pumps, poor surface turbulence which leads to slugging of high pressure pumps and non-uniform slurry density, poor wetting of the new sand due to the problems of getting clean fluid and sand together without excessive agitation, the stacking of dry sand on the sides of the slurry tub, sealing of agitators to prevent fluid loss and the lack of available suction head at the centrifugal pumps.
There is a need for a mixing system particularly adapted for the effective mixing of high density sand slurries for well fracturing purposes.